A webapp is like a mobile website but with the look and feel of a native app. To open a webapp you have to know and enter its url on your mobile browser, or you can also flash a QR Code. Unlike native apps you can't access to device features like camera, GPS, adress book and others. It's less efficient than a native app but it's compliant with all mobile browsers and so with all mobile devices. A webapp or mobile website can also be find by search engines.
Native apps and Mobile websites are complementary. Native app is good for CRM and loyalty as it's faster and more efficient, but mobile website is good for notoriety as it can be ranked in search engines.